Electrostatic actuators are known in which an electrostatic force acts between a stator and a mover included in an actuator, and the attraction force thereof is used to drive the mover. Further, so-called semiconductor processes (the manufacturing technology of semiconductor devices) have been used to develop extremely small electrostatic actuators in MEMS (Micro Electro Mechanical Systems) and the like.
Also in MEMS and the like, so-called MEMS switches that utilize electrostatic actuators are known. There are such known MEMS switches in which a film body unit is suspended by multiple spring components; when the switch is ON, a pull-in voltage is provided and the film body unit is electrostatically attracted to the electrode unit against a spring force of the spring components; and when the switch is OFF, a pull-out voltage is provided and the film body unit is caused to separate from the electrode unit by the spring force of the spring components.
Electrostatically-driven MEMS switches generally have a high pull-in voltage of, for example, not less than 20 V (volts). Therefore, in the case where a MEMS switch is used in a mobile system such as a mobile telephone, a voltage step-up circuit is necessary. In such a case, it is disadvantageous for the mobile system because the voltage step-up circuit not only has a large chip surface area but also uses a large current. Moreover, noise occurring in the voltage step-up circuit may cause misoperations of the wireless circuit.
In such a case, reducing the rigidities of the spring components can reduce the pull-in voltage. However, in the case where the rigidities of the spring components are simply reduced, a discrepancy in which the film body unit and the electrode unit remain in contact and do not separate, i.e., a so-called stiction defect, occurs easily. Further, reducing the pull-in voltage weakens the force with which the film body unit and the electrode unit contact each other, i.e., the contact force. As a result, there is a risk that the contact resistance of the switch may undesirably increase.
Such problems exist not only in contact-type MEMS switches but also similarly in variable capacitors and the like used in high frequency circuits. In other words, simply reducing the rigidities of the spring components to reduce the pull-in voltage causes stiction defects to occur easily. Also, there is a risk that a large capacitance ratio cannot be obtained because the contact force is weakened.
Therefore, technology has been proposed to suspend the film body unit using multiple spring components having different rigidities, electrostatically attract the portion of the film body unit from the side where the spring components having low rigidities are provided in the ON state, and separate from the portion of the film body unit on the side where the spring components having high rigidities are provided in the OFF state (refer to JP-A 2007-35641 (Kokai)).
However, in the technology discussed in JP-A 2007-35641 (Kokai), there is no consideration for attracting the film body unit to the electrode unit by a small force when starting the attraction; and there is a risk of unstable operations when starting the attraction.
Further, because the spring components are provided on the short sides of a film body unit having a rectangular shape, the film body unit flexes easily between the opposing spring components. Therefore, there is a risk of unstable operations during separation when the switch is OFF such as the film body unit flexing and a portion of the film body unit not separating from the electrode unit, etc.